The present invention relates generally to imaging systems employing a microbolometer focal plane array. More specifically, the present invention is directed to a method and circuitry for correcting temperature-induced errors by way of a controlled bias.
An infrared detector called the xe2x80x9cbolometer,xe2x80x9d now well known in the art, operates on the principle that the electrical resistance of the bolometer material changes with respect to the bolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics can be exploited to measure incident infrared radiation on the bolometer by sensing the resulting change in its resistance. When used as an infrared detector, the bolometer is generally thermally isolated from its supporting substrate or surroundings to allow the absorbed incident infrared radiation to generate a temperature change in the bolometer material.
Modern microbolometer structures were developed by the Honeywell Corporation. For a recent summary of references see U.S. Pat. No. 5,420,419. A two-dimensional array of closely spaced microbolometer detectors is typically fabricated on a monolithic silicon substrate or integrated circuit. Commonly, each of these microbolometer detectors are fabricated on the substrate by way of an, commonly referred to, air bridge structure which includes a temperature sensitive resistive material element that is substantially thermally isolated from the substrate. This aforesaid microbolometer detector structure is herein referred to as a xe2x80x9cthermally-isolated microbolometer.xe2x80x9d The resistive element, for example may be vanadium oxide that absorbs infrared radiation. The constructed air bridge structure provides good thermal isolation between the resistive element of each microbolometer detector and the silicon substrate. An exemplary microbolometer structure may dimensionally be in the order of approximately 50 microns by 50 microns.
In contrast, a microbolometer detector that is fabricated directly on the substrate, without the air-bridge structure, is herein referred to as a xe2x80x9cthermally shorted microbolometer,xe2x80x9d since it will be directly affected by the temperature of the substrate and/or package. Alternately, it may be regarded as a xe2x80x9cheat sunkxe2x80x9d pixel since it is shorted to the substrate.
Microbolometer detector arrays may be used to sense a focal plane of incident radiation (typically infrared). Each microbolometer detector of an array may absorb any radiation incident thereon, resulting in a corresponding change in its temperature, which results in a corresponding change in its resistance. With each microbolometer functioning as a pixel, a two-dimensional image or picture representation of the incident infrared radiation may be generated by translating the changes in resistance of each microbolometer into a time-multiplexed electrical signal that can be displayed on a monitor or stored in a computer. The circuitry used to perform this translation is commonly known as the Read Out Integrated Circuit (ROIC), and is commonly fabricated as an integrated circuit on a silicon substrate. The microbolometer array may then be fabricated on top of the ROIC. The combination of the ROIC and microbolometer array is commonly known as a microbolometer infrared Focal Plane Array (FPA). Microbolometer FPAs containing as many as 640xc3x97480 detectors have been demonstrated.
Individual microbolometers will have non-uniform responses to uniform incident infrared radiation, even when the bolometers are manufactured as part of a microbolometer FPA. This is due to small variations in the detectors"" electrical and thermal properties as a result of the manufacturing process. These non-uniformities in the microbolometer response characteristics must be corrected to produce an electrical signal with adequate signal-to-noise ratio for image processing and display.
Under the conditions where a uniform electric signal bias source and incident infrared radiation are applied to an array of microbolometer detectors, differences in detector response will occur. This is commonly referred to as spatial non-uniformity, and is due to the variations in a number of critical performance characteristics of the microbolometer detectors. This is a natural result of the microbolometer fabrication process. The characteristics contributing to spatial non-uniformity include the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors.
The magnitude of the response non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation. The resulting ROIC output signal is difficult to process, as it requires system interface electronics having a very high dynamic range. In order to achieve an output signal dominated by the level of incident infrared radiation, processing to correct for detector non-uniformity is required.
Previous methods for implementing an ROIC for microbolometer arrays have used an architecture wherein the resistance of each microbolometer is sensed by applying a uniform electric signal source, e.g., voltage or current sources, and a resistive load to the microbolometer element. The current resulting from the applied voltage is integrated over time by an amplifier to produce an output voltage level proportional to the value of the integrated current. The output voltage is then multiplexed to the signal acquisition system.
Gain and offset corrections are applied to the output signal to correct for the errors that may arise from the microbolometer property non-uniformities. This process is commonly referred to as two-point correction. In this technique two correction coefficients are applied to the sampled signal of each element. The gain correction is implemented by multiplying the output voltage by a unique gain coefficient. The offset correction is implemented by adding a unique offset coefficient to the output voltage. Both analog and digital techniques have been utilized to perform this two-point non-uniformity correction.
The current state-of-the-art in microbolometer array ROICs suffers from two principal problems. The first problem is that the large magnitude of the microbolometer introduced non-uniformities in the ROIC output signal requires a large instantaneous dynamic range in the sensor interface electronics, increasing the cost and complexity for the system. Current advanced ROIC architectures incorporate part of the correction on the ROIC to minimize the instantaneous dynamic range requirements at the acquisition systems interface.
The second problem is that the application of a fixed coefficient two-point gain and offset correction method to minimize array non-uniformity works well only for a very small range of substrate temperatures, on the order of 0.005 to 0.025 degrees Kelvin. In order to maintain the substrate temperature within this range, a thermoelectric cooler, temperature sensor, and temperature control electronics are required, again adding to system cost and complexity.
Application of the thermoelectric cooler directly reduces the need for a large instantaneous dynamic range in the sensor interface electronics. This is so, since infrared induced temperatures changes in the microbolometer detector are very small as compared to induced temperature change in the microbolometer detector due to changes in substrate temperature from heat produced by the accompanying electronics as well as the operating environment temperature.
An uncooled microbolometer focal plane array is taught in U.S. Pat. No. 5,756,999, entitled, xe2x80x9cMethods and Circuitry for Correcting Temperature-induced Errors in Microbolometer Focal Plane Array, issued to Parrish, et. al. Therein a bias correction method and a pre-bias correction method are taught. With regard to the bias correction method, as stated therein, xe2x80x9cAccording to the bias-correction method, a unique bias amplitude is applied to each detector during the integration period to support uniformity correction. The bias-correction method can be implemented as an adjustable voltage, current, or load bias that is applied to the microbolometer detectors during the integration (measurement) period . . . the bias-correction value is applied during the integration period of the microbolometer detector using an adjustable voltage source . . . The bias-correction value is controlled by the output of a digital-to-analog converter (DAC) . . . The adjustable bolometer bias may be used to correct the optical gain of the signal for uniform output at a particular substrate temperature in conjunction with single point offset correction . . . to remove residual fixed offsets.xe2x80x9d
Although the aforesaid patent sets forth improved methods for correcting temperature induced errors in microbolometer focal plane arrays, these methods still remain complex and time consuming.
An object of the present invention is to provide a simple method for correcting for temperature induced errors in microbolometer focal plane arrays.
Another object of the invention is to provide a simple bias correction scheme to minimize the need for a large dynamic range in the sensor interface electronics.
In accordance with the present invention, a single thermally-shorted microbolometer detector is employed for controlling the magnitude of an electric signal bias source that is applied to all microbolometers of a focal plane array on a substrate. A calibration bias source magnitude is determined and continually adjusted as a function of the reading value of the resistance of the thermally-shorted microbolometer at calibration, and readings of the thermally-shorted microbolometer after each image sample is taken.